The present invention relates to a car body coating process in which a shell body is formed using plastics-covered metal plates to a portion or whole of the shell body of a car body and the exposed metal portion of the shell body is coated by electrodeposition.
In a car body of a normal or small passenger car, the portion constituted mainly with sheet metal without riggings such as engine or chassis is called shell body which is constituted usually with a main body consisting of underbody, side member, roof, cowl, upper back, lower back etc. and outer cover parts such as hood, front balance, front fender, cowl louver, door, luggage (back door) etc.
Up to the present, in order to form these main body and outer cover parts, metal plate is cut and molded in the size and form of each constituting part and the parts are assembled to a shell body which is dipped into an electrodeposition paint bath to form a primer film on the surface, backside, edge surface portion etc. by electrodeposition coating. Intermediate paint, topcoat paint etc. are then coated to these outer portions.
Recently, however, operation step saving, energy saving and CO2 reduction in the coating line is strongly desired also in the coating of a car body. Moreover, further improvement in throwing power of electrodeposition paint to edge surface portion, chipping resistance of the total coating film, corrosion resistance etc.
In the coating of a car body, on the other hand, there is proposed an inverse method (for example, Japanese Patent Publication No. 41317/1980), in which a substrate is powder-coated and the uncoated portion is coated by electrodeposition, for the purpose of the reduction of solvent discharge and improvement of edge surface coatability of electrodeposition paint. This method has, however, such problems as that powder coating facilities become additionally necessary, and that the corrosion resistance of the boundary portion between powder coating film and electrodeposition coating film is not sufficient.
The present inventors have repeatedly conducted studies to solve the above-mentioned problems in the coating of a car body. As a result, this time, they found that the above-mentioned problems can be solved by one effect by forming a shell body by using a metal plate covered with plastics layer, which has been prepared previously, to a portion or whole of the shell body of a car body, and coating the exposed metal portion in the shell body by electrodeposition.
Thus, according to the present invention there is provided a car body coating process characterized by forming a shell body using a plastics-covered metal plate to the main outer parts of a car body, and then coating the exposed metal portion in the shell body by electrodeposition (this process is hereinafter referred to as Process I).
According to the present invention there is provided also a car body coating process characterized by forming a shell body equipping car parts, which have been made by cutting, molding and joining a plastics-covered metal plate for the main outer portions of a car body, to a previously assembled main body of a car body, and then coating the exposed metal portion in the shell body by electrodeposition (this process is hereinafter referred to as Process II).
Then, the car body coating process of the present invention is described in more detail.
The process of the present invention can be applied mainly to normal and small passenger car, but can be also applied to truck, bus, motorcycle, vehicle with special kind of equipment car etc. in the same manner.
As a metal plate to be covered with plastics, metals, which have been used for a car body up to the present, can be similarly used. As the material, there may be mentioned, for example, iron, steel, stainless steel, aluminum, copper, and alloys containing these metals, and further plate of these metals whose surface is plated with zinc, zinc/nickel, iron etc. They can be used usually in coils or processed in cut plates. The thickness of these metal plates is suitably in the range of generally 0.3-2.0 mm, particularly 0.5-1.0 mm and the surface of these metal plates is preferable to be treated suitably by grinding, degreasing, with phosphate etc. in order to improve the adhesivity with a plastics layer, corrosion resistance etc.
As plastics materials to cover the above-mentioned metal plates, there can be used per se known thermoplastic resins, for example, polyolefin resin such as polyethylene, polypropylene etc., polyester resin such as polyethylene terephthalate (PET) etc., polycarbonate resin, epoxy resin, vinyl acetate resin, vinyl chloride resin, fluorine-containing resin, polyvinyl acetal resin, polyvinyl alcohol resin, polyamide resin, polystyrene resin, acrylic resin, polyurethane resin, phenolic resin, polyether resin, cellulose type resin etc. They may contain color pigment, extender pigment etc.
Covering of a metal plate with these plastics materials can be performed by per se known methods including, for example, sticking onto the metal plate plastics in film or sheet formed by the methods such as extrusion molding, injection molding, calender molding, compression molding etc.; sticking with pressure onto the metal plate molten plastics extruded in film or sheet; adhering plastics in powder form onto the metal plate by the methods such as fluidized immersion, electrostatic coating etc. and then melting by heating. Particularly the process of sticking plastics in film or sheet onto the metal plate is preferable. Covering of a metal plate with plastics is performed at least to the surface of a metal plate located at outer side of a car body. However, it is possible to cover both surfaces, if desired. The thickness of the plastics layer covering a metal plate is preferable in the range of usually 1-100 xcexcm, particularly 3-75 xcexcm, more particularly 5-50 xcexcm. Moreover, it is possible to treat the surface of these plastics by corona discharge, plasma, flame etc. before or after the covering onto the metal plate.
On covering a metal plate with plastics, in case of sticking plastics in film or sheet onto the metal plate, it is preferable to previously coat an adhesive to the metal plate and/or plastics film or sheet, in order to increase adhesivity between the two. As such an adhesive there may be mentioned thermosetting or thermoplastic adhesives containing one or more kinds of resins selected from, for example, bisphenol type epoxy resin, resol type epoxy resin, acrylic resin, aminoplast resin, polyester resin, urethane resin, polysiloxane resin etc., and further containing optionally a curing agent. Further, there can be used as adhesive triazinethiol type compounds such as 2,4, 6-trimercapto-S-triazine, 2-dibutylamino-4, 6-dimercapto-S-triazine, 2,4,6-trimercapto-S-triazine-monosodium salt, 2,4,6-trimer-capto-S-triazine-trisodium salt etc.
In the Process I according to the present invention, a shell body is assembled by cutting, molding and joining the plastics-covered metal plates, which have been prepared as mentioned above. Specifically each part of the main body and outer cover parts is prepared using the plastics-covered metal plate, and then the parts are assembled to form a shell body.
Shell body is a portion in a car body, constituted mainly with sheet metal without riggings such as engine or chassis. Its main body is constituted mainly with parts such as underbody, side member, roof, cowl, upper back, lower back etc. and the outer cover parts consists mainly of parts such as hood, front balance, front fender, cowl louver, door, luggage (back door) etc. Parts of outer cover parts are called car parts.
Underbody here means the floor portion of the cabin, trunk room etc. and is named generically, including front underbody, front floor, rear floor etc. Side member forms the side of a cabin joining with a front body, roof panel, underbody etc. and prevents the car from bending and/or twisting. Cowl is a panel combining left, right, front and rear pillars. Upper back is a panel combining left and right quarter panels (rear fender) at the back portion of a car body and forming outer surface of the car body.
In the Process I of the present invention, in order to form the above-mentioned parts constituting a shell body, a plastics covered metal plate, which has been prepared as mentioned above, is cut to the suitable shape and size, pressed and molded by a press etc., and joined by, as necessary, adhering with adhesive, welding, bolted etc. to prepare parts of the main body such as underbody, side member, roof, cowl, upper back, lower back etc.; and parts (car parts) of outer cover parts such as hood, front balance, front fender, cowl louver, door, luggage etc. These cutting, molding and joining can be performed by per se known methods. Then the parts of the main body thus formed using a plastics-covered metal plate are assembled and joined to form a main body, to which parts of outer cover parts (car parts) such as hood, front balance, front fender, cowl louver, door, luggage etc. are equipped.
While at least the outer surface of the shell body thus assembled using a plastics-covered metal plate is covered with a plastics layer, the edge surface portion of a cut plastics-covered metal plate has an exposed metal portion. Moreover, though the back side is preferably covered with plastics, a metal portion may be exposed. In the Process I of the present invention these exposed metal portions are then coated by electrodeposition.
On the other hand, in the Process II according to the present invention, parts of outer cover parts (car parts) such as hood, front balance, front fender, cowl louver, door, luggage etc. are prepared, using a plastics-covered metal plate, which has been prepared as mentioned above, by cutting, molding and joining them, and these car parts are equipped to a previously assembled main body of a car body to form a shell body. Among them, the preparation of parts of outer cover parts (car parts) can be performed in the same manner as in the above-mentioned Process I.
In the Process II of the present invention, most or the whole of the car parts, which constitute outer cover parts, are prepared using the above-mentioned plastics-covered metal plate. For example, in order to form each part constituting outer cover parts such as hood, front balance, front fender, cowl louver, door, luggage (back door) etc., a plastics-covered metal plate is cut to the suitable shape and size, pressed and molded by a press etc., and joined by, as necessary, adhering with adhesive, welding, bolting etc. to prepare parts (car parts) such as hood, front balance etc. These cutting, molding and joining can be performed by per se known methods. At least the outer surface of the parts of outer cover parts (car parts) thus formed is covered with a plastics layer and the edge surface portion of a cut steel plate has an exposed metal portion. The back side may be uncovered and have exposed metal, or may be covered with plastics.
In the Process II of the present invention, the main body constituted with underbody, side member, roof, cowl, upper back, lower back etc., to which these car parts are equipped, are prepared usually, without using a plastics-covered metal plate but using an uncovered metal plate, by cutting, molding and processing, and joining them by per se known methods. A shell body is formed by equipping the parts of outer cover parts (car parts) prepared using a plastics-covered metal plate to a main body prepared using such an uncovered metal plate. In the Process II of the present invention, the whole surface of the main body and the exposed metal portion of outer cover parts (car parts) in thus assembled shell body are coated by electrodeposition.
Electrodeposition paint to be used for the electrodeposition coating of the assembled shell body in the Processes I and II of the present invention may be either anionic type or cationic type. Generally, however, it is preferable to use a cationic type electrodeposition paint with excellent corrosion resistance.
As a cationic electrodeposition paint a known product can be used, for example, a water paint containing a base resin having a hydroxyl group(s) and a cationizable group(s) (a) and a blocked polyisocyanate compound (b).
As a base resin (a), for example, the following can be mentioned.
1) reaction product of epoxy resin and cationizing agent; 2) acid-protonized product of polycondensate of polycarboxylic acid and polyamine (cf. U.S. Pat. No. 2,450,940 Specification); 3) acid-protonized product of polyadduct of polyisocyanate compound, polyol and mono- or polyamine; 4) acid-protonized product of copolymer of acryl type or vinyl type monomers having hydroxyl group and amino group (cf. Japanese Patent Publications No. 12395/1970 and No. 12396/1970); 5) acid-protonized product polyadduct of polycarboxylic acid resin and alkyleneimine (cf. U.S. Pat. No. 3,403,088 Specification).
As specific examples and preparation processes of these base resins (a) are described, for example, in Japanese Patent Publications No. 12395/1970, No. 12396/1970, No. 23087/1974, U.S. Pat. No. 2,450,940, 3,403,088, U.S. Pat. No. 3,891,529, U.S. Pat. No. 30 3963663 etc., these literatures are quoted here instead of a detailed description.
Among these, a resin, obtained by reacting a cationizing agent to an epoxy resin, which is obtained by a reaction of poly-phenol compound and epichlorohydrin, and included in the above-mentioned 1), is particularly preferable due to its formation of a 5 coating film with excellent corrosion resistance.
As the above-mentioned epoxy resin it is particularly suitable to have more than 2 epoxy groups in the molecule, number average molecular weight of more than 200, preferably 800-2000, and epoxy equivalent in the range of 190-2000, preferably 400-1000. Such epoxy resins include polyglycidyl ether of polyphenol compound. As said polyphenol compound there can be mentioned, for example, bis(4-hydroxyphenyl)-2,2-propane, 4,4xe2x80x2-dihydroxybenzo-phenone, bis(4-hydroxyphenyl)-1,1-ethane, bis(4-hydroxyphenyl)-1,1-isobutane, bis(4-hydroxy-tert-butylphenyl)-2,2-propane, bis(2-hy-15 droxybutyl)methane, 1,5-dihydroxynaphthalene, bis(2,4-dihydroxyphenyl) methane, tetra(4-hydroxyphenyl)- 1,1,2,2-ethane, 4,4xe2x80x2-dihy-droxydiphenyl ether, 4,4xe2x80x2-dihydroxydiphenyl sulfone, phenol novolac, cresol novolac etc.
These epoxy resins may be further modified by reacting with polyol, polyehter polyol, polyester polyol, polyamidoamine, polycarboxylic acid, polyisocyanate compound etc. and further may be graft-polymerized by e-caprolactone, acrylic monomer etc.
Cationizing agent in the above-mentioned 1) gives a base resin (a) having a hydroxyl group(s) and a cationazable group(s) by reacting with most or whole of the epoxy groups existing in the epoxy resin and introducing cationizable group such as secondary amino group, tertiary amino group, quaternary ammonium base etc. into the resin.
As such a cationazing agent there can be mentioned amine compound, for example, primary amine, secondary amine, tertiary amine, polyamine etc. Here, there can be mentioned as a primary amine compound, for example, methylamine, ethylamine, n-propylamine, isopropylamine, monoethanolamine, n-propano-lamine, isopropanolamine, etc.; as a secondary amine compound, for example, diethylamine, diethanolamine, di-n-propanolamine, diisopropanolamine, N-methylethanolamine, N-ethylethanolamine etc.; and as a tertiary amine compound, for example, triethylamine, triethanolamine, N,N-dimethylethanolamine, N-methyldiethanol-amine, N,N-diethylethanolamine, N-ethyldiethanolamine etc. As a polyamine there can be mentioned, for example, ethylenediamine, diethylenetriamine, hydroxyethylaminoethylamine, ethylamino-ethylamine, methylaminopropylamine, dimethylaminoethylamine, dimethylaminopropylamine etc.
Furthermore, it is possible to use a basic compound such as ammonia, hydroxylamine, hydrazine, hydroxyethylhydrazine, N-hydroxyethylimidazoline etc. as a cationazing agent to react with an epoxy group and to protonize the basic group formed therby with an acid to make it into a cationizable group. As an acid usable here a water-soluble organic carboxylic acid, for example, formic acid, acetic acid, glycolic acid, lactic acid etc., is preferable.
As a hydroxyl group in the base resin (a) there may be mentioned a primary hydroxyl group introduced by, for example, a reaction with an alkanol amine in the above-mentioned cationizing agent, a ring-opening reaction with caprolactone which may be introduced in the epoxy resin, a reaction with a polyol, etc.; a secondary hydroxyl group in the epoxy resin etc. Among them a primary hydroxyl group introduced by a reaction with an alkanolamine is preferable due to its excellent crosslinking reactivity with a blocked polyisocyanate compound (crosslinking agent). The amount of hydroxyl groups in the base resin (a) is preferably in the range of generally 20-5000 mgKOH/g, particularly 100-1000 mgKOH/g. Particularly it is preferable that the primary hydroxyl group equivalent be in the range of 200-1000 mgKOH/g as a hydroxyl group equivalent. The amount of a cationizable group is preferable to be more than the minimum limit necessary to stably disperse the base resin (a) in water and preferable in the range of generally 3-200, particularly 10-80 calculated as KOH (mg/g solid content) (amine value). It is desirable that the base resin (a) does not substantially contain a free epoxy group.
Blocked polyisocyanate compound (b) as a crosslinking agent is a compound in which substantially all isocyanate groups in the polyisocyanate compound are blocked by a volatile blocking agent. Upon heating it over the prescribed temperature the blocking agent is dissociated to regenerate an isocyanate group which takes part in the crosslinking reaction with the base resin (a).
Polyisocyanate compound is a compound having more than 2 free isocyanate groups in the molecule and includes, for example, aliphatic diisocyanate such as hexamethylene diisocyanate, trimethylene diisocyanate, tetramethylene diisocyanate, dimer acid diisocyanate, lysine diisocyanate etc.; alicyclic diisocyanate such as isophorone diisocyanate, methylenebis(cyclohexylisocyanate), methylcyclohexane diisocyanate, cyclohexane diisocyanate, cyclopentane diisocyanate etc.; aromatic diisocyanate such as xylylene diisocyanate, tolylene diisocyanate, diphenylmethane diisocyanate, naphthalene diisocyanate, toluidine diisocyanate etc.; urethanation adducts, biuret type adducts, isocyanuric ring type adducts of these polyisocyanate compounds etc.
As a blocking agent to block a free isocyanate group of these polyisocyanate compounds there can be used known blocking agents of phenol type, alcohol type, active methylene type, mercaptan type, acid amide type, imide type, amine type, imidazole type, urea type, carbamic acid type, imine type, oxime type, sulfurous acid type, lactam type etc.
Constitution ratio of the base resin (a) and the blocked polyisocyanate compound (b) is preferably for the former in the range of 40-90%, particularly 50-80%, and for the latter, 60-10%, particularly 50-20% based upon the total solid content weight of both components.
A cationic electrodeposition paint can be prepared, for example, by neutralizing a cationizable group in a base resin (a) with an acid compound such as acetic acid, formic acid, lactic acid, phosphoric acid etc. and then mixing in water together with a blocked polyisocyanate compound (b). The pH at the time of its coating is suitably in the range of generally 3-9, particularly 5-7 and the solid content concentration is suitably in the range of 5-30% by weight.
To a cationic electrodeposition paint there can be suitably compounded, as necessary, a curing catalyst having rust preventive properties such as hydroxide, oxide, organic acid salt, inorganic acid salt etc. of a metal selected from aluminum, nickel, zinc, strontium, lead, zirconium, molybdenum, tin, antimony, lanthanum, tungsten, bismuth etc.; extender pigment, color pigment, rust preventive pigment, antisettling agent etc.
In the present invention, an electrodeposition coating film can be deposited to an exposed metal portion in the shell body, for example, edge surface portion of a cut plastics-covered metal plate and the backside portion of said metal plate which is not covered with plastics, or the whole surface of the main body (in case of Process II) etc. by dipping a shell body prepared as mentioned above into a cationic electrodeposition paint bath, making it a cathode and conducting an electrodeposition coating for 1-10 minutes of passing a current, at 20-35xc2x0 C. of bath temperature and 100-400 V of voltage. The thickness of an electrodeposition coating film is preferably in the range of usually about 10xe2x80x94about 40 xcexcm, particularly 10-20 xcexcm as a cured film. After the coating, the shell body is drawn up from the electrodeposition paint bath, washed suitably with water and heated to about 100 xe2x80x94about 200xc2x0 C. to cure the electrodeposition coating film and thus to obtain a car body by the present invention.
Through the process of the present invention as mentioned above the following effects can be obtained.
(1) As the electrodeposition paint deposits easily at the boundary portion to the plastics covering film at the portion with exposed metal as a thick film, the corrosion resistance of this portion improves remarkably.
(2) As the main body, consisting of outer cover parts such as hood panel, fender panel, door panel, luggage door panel etc. of the car body and further underbody, side member, roof, cowl, upper back, lower back etc., can be prepared using metal plates which have been previously covered with plastics, it is possible to largely reduce the amount of the electrodeposition paint to be used at the next step.
(3) As at least the outer surface of outer cover parts is covered with a layer of plastics having a high volume specific resistance and, optionally, at least the outer surface of the main body is also covered with a layer of plastics having a high volume specific resistance, the area of the portion of shell body to be coated by electrodeposition (portion with exposed metal) is small and consequently the throwing power of the paint increases and particularly the corrosion resistance of the edge surface portion is improved.
(4) It is possible to give the properties, which the covering plastics have, such as chipping resistance, corrosion resistance etc. to a car body.